Splicing the active phases of copper/cobalt-based catalysts achieves high-rate tandem electroreduction of nitrate to ammonia

Abstract

Electrocatalytic recycling of waste nitrate (NO 3−) to valuable ammonia (NH 3) at ambient conditions is a green and appealing alternative to the Haber−Bosch process. However, the reaction requires multi-step electron and proton transfer, making it a grand challenge to drive high-rate NH 3 synthesis in an energy-efficient way. Herein, we present a design concept of tandem catalysts, which involves coupling intermediate phases of different transition metals, existing at low applied overpotentials, as cooperative active sites that enable cascade NO 3−-to-NH 3 conversion, in turn avoiding the generally encountered scaling relations. We implement the concept by electrochemical transformation of Cu−Co binary sulfides into potential-dependent core−shell Cu/CuO x and Co/CoO phases. Electrochemical evaluation, kinetic studies, and in−situ Raman spectra reveal that the inner Cu/CuO x phases preferentially catalyze NO 3− reduction to NO 2−, which is rapidly reduced to NH 3 at the nearby Co/CoO shell. This unique tandem catalyst system leads to a NO 3−-to-NH 3 Faradaic efficiency of 93.3 ± 2.1% in a wide range of NO 3− concentrations at pH 13, a high NH 3 yield rate of 1.17 mmol cm−2 h−1 in 0.1 M NO 3− at −0.175 V vs. RHE, and a half-cell energy efficiency of ~36%, surpassing most previous reports.